The invention is an inductive heating apparatus and method for heating an inductively reactive heat radiating element built into a heat retentive food server warming plate. Warming plates are also known in the industry as food service containers and are typically used in hospitals and restaurants for maintaining the appropriate serving temperature of prepared food.
Warming plates and devices for heating them are not unknown. For example, U.S. Pat. No. 3,684,862 discloses a thermostatically controlled electroresistive cooker hot plate having an temperature sensor for controlling the temperature of objects that are placed on the surface of the hot plate. The '862 patent does not disclose inductive heating, object detection by means of optical or inductive load sensing, foreign object discrimination, or temperature control by inductor current flow. In U.S. Pat. No. 4,757,176, an induction heating cooker control circuit is disclosed having a diode bridge rectifier, conditioning capacitors and inductors, a resonant heating coil circuit, a switching transistor and diode circuit, and oscillation circuitry for control and regulation of heat produced by an induction coil. The patent appears to provide for temperature sensing by means of a thermistor temperature sensor. However, unlike the present invention which is described below, the patent does not disclose the detection of an object by use of optical or inductive load sensing, foreign object discrimination, or temperature control by inductor current flow.
U.S. Pat. No. 5,136,277 discloses a cooking hob having a method and apparatus for detecting the presence and measuring the dimensions of an object on the surface of a glass ceramic hob. The patent discloses the control of halogen lamp heating elements on the basis of the measured dimensions of the detected object. In the '277 patent, an object is detected by a disturbance of an electric field that is generated by a conductive element that is separate and distinct from a heating element. However, unlike the preset invention, this patent does not disclose the incorporation of inductive load sensing or object discrimination by use of an induction coil and control circuitry.
In U.S. Pat. No. 5,424,512 a capacitive sensing device is disclosed for detecting the presence of a food cooking container on a cooking hob. The capacitive sensing device has a fixed capacitance that is established by two conductive plates of opposite polarities which are located under the surface of the hob. When a food container is placed on the surface, the fixed capacitance is changed, thereby causing the heating element, a resistive element or gas burner, to be activated according to parameters stored in a microprocessor. However, unlike the present invention, the patent does not disclose the detection of an object by use of optical or inductive load sensing, foreign object discrimination, or temperature control by inductor current flow.
U.S. Pat. No. 5,603,858 discloses a heat retentive server having a central heat storage member susceptible to electrical induction heating. Although the '858 patent describes a heat retentive server, it does not disclose in any detail a circuit or method for non-contact detection and inductive heating of heat retentive food server warming plates. In particular, this patent does not disclose object detection by means of optical or inductive load sensing, foreign object discrimination, or temperature control by inductor current flow.
U.K. patent application GB 2,171,567 discloses induction heating of electric plates of a cooker by applying high frequency power pulses greater than 20 kHz to an induction coil located under the surface of a cooker. Before power is applied to the induction coil, a ferromagnetic container or body must be placed onto the surface of the cooker. Detection of a ferromagnetic container is disclosed as the dynamic detection of the recovery time of the inductive energy within the induction coil. However, unlike the present invention, the patent does not disclose the detection of an object by use of optical, foreign object discrimination, or temperature control by inductor current flow.
The attempts made in the past to use induction heating to heat up hot plates for use in institutional food service are known to have many problems. One major problem with current systems is that a user can overheat a hot plate or initiate a heating cycle more than once and damage the hot plate. This present invention solves this problem by means of a novel control logic employing electronic signals such as a current feedback in a control loop of the induction heating unit to detect the difference between cold and heated plate. The electronic signals do not directly measure or detect the actual temperature of the plate being heated. Instead, the design relies upon the change in physical properties such as impedance and electrical resistance of the plate as it is heated to detect the difference in temperature between a cold and a heated plate. This design provides a more reliable turn-key go or no go system. For example, when a plate is cold it will be heated and a plate previously heated will not be heated.
The system allows the user to establish preselected heating parameters such as a current feedback value and sampling time for a hot plate or series of hot plates. In addition, the system may have pre-stored current feedback values and sampling times for any number of different hot plates and heat them at different rates and temperatures.
Although many attempts have been made in the prior art to produce a reliable and efficient system for heating warming plates for maintaining the serving temperature of food in the food service industry, none have solved the problem or addressed the need for an apparatus and method for non-contact detection and the control of inductive heating of heat retentive food server warming plates.